At least wait a month or two until the weather warms up. BMW Z3 2.8is are great lookers and future classics but haven't seen big rises yet, so if you want to make money on it you'll need to keep it a couple more years.

I don't think it will make big money at auction yet, but advertising privately there's a chance you'll find a punter who just has to have it. Be very careful about uninsured test drives and about taking the money. Cash, handed over in a bank, where you can immediately bank it, is best.